Middleware Pipeline

What is it?

The ASP.NET framework implements a modular pipeline that is executed between the HTTP request and the server response. Think of it as the entry point of the application. This is the middleware pipeline.

The middleware pipeline contains one or more middleware, each executing an action, calling the next middleware, executing another action while awaiting the response, and then returning the response.

Tip

Each middleware component can be thought of as a separate Service. For example, you can use the .AddMvc() service to add this functionality to your application.

Some middleware don’t implement the next() method, resulting in not calling a next middleware. These middleware are referred as a terminal middleware, most used with the .Run() method.

Defining a middleware pipeline, that implements asynchronous messages, with each middleware calling the next until the terminal middleware.

// Set WebApplicationBuilder  
var builder = WebApplication.CreateBuilder(args);  
  
// Set to building  
var myApp = builder.Build();  
  
void ConfigureMiddleware(IApplicationBuilder app)  
{  
    //  From the HTTP context, defines the first middleware and invokes the second  
    app.Use(async (context, next) =>  
    {  
        await context.Response.WriteAsync($"First mid \n");  
        await next.Invoke();  
        await context.Response.WriteAsync("Second mid \n");  
    });  
      
    app.Run(async context => {  
        // Terminal middleware. Gets requested path and writes in the message  
        await context.Response.WriteAsync($"Hello from {context.Request.Path} \n");  
    });  
}  
  
// Applies configuration  
ConfigureMiddleware(myApp);  
  
// Runs the WebApp.  
myApp.Run();

Short-circuiting the pipeline

Every middleware that is not terminal, should invoke the next middleware. If not, then it will short-circuit the pipeline. At this point, the pipeline will stop running and only the middlewares already called will respond.